Fixed a number of issues with the changes to fetchart:
- Remove redundant fetches. This was making the Amazon source download every
image twice even when art resizing was not enabled!
- Restore local_only switch in plugin hook, which got lost in the shuffle at
some point.
- Don't replace the original image file in-place; use a temporary file instead.
This would clobber the original source image on the filesystem with the
downscaled version!